It is an offence to use your mobile phone if supervising a provisional driver.
As the person instructing a learner driver is legally responsible for the car, road laws that would normally apply to a driver also apply to the instructor in the passenger seat. That means it's illegal for the instructor to touch their mobile phone in any way while teaching another person to drive. This applies to both professional driving instructors, and friends or family of a new driver who may be supervising a lesson.
The same rule also applies for the drink-driving limit, with all instructors required to be under the legal maximum while in the front passenger seat with a learner at the wheel.
In most cases, supervisors should treat a lesson as if they are driving themselves - but not all traffic offences will affect their licence. For example, if the learner breaks the speed limit while under supervision, their licence will receive the fine and penalty points instead of the instructor.